Enhanced Anaerobic Biodegradation through Sulfate Reduction Induced by the Addition of Nitrate and Phosphate

Transcription

1 Enhanced Anaerobic Biodegradation through Sulfate Reduction Induced by the Addition of Nitrate and Phosphate Chris Mathies and Wenhui Xiong (Stantec Consulting Ltd., Saskatoon, Saskatchewan, Canada) Kris Bradshaw, Trevor Carlson, and Kimberley Tang (Federated Co-operative Ltd., Saskatoon, Saskatchewan, Canada) ABSTRACT: The project team designed an enhanced anaerobic biodegradation (EAB) program for a site which contained abundant levels of naturally occurring sulfate. It was hypothesized that the introduction of nitrate as a supplied electron acceptor and phosphate as a macronutrient could stimulate nitrate reduction, which would facilitate the sequential transition to sulfate reducing conditions once the nitrate had been preferentially consumed. This concept was designed to leverage existing site conditions (high sulfate concentrations) and cost-effectively implement an EAB program to increase the degradation rates of petroleum hydrocarbons (PHC), following a mechanical recovery program. PHC soil and groundwater data confirmed that the EAB process effectively reduced BTEX concentrations. Benzene, which is generally one of the more difficult compounds to biodegrade, was documented to have measurable removal efficiencies. Nitrate concentrations during this time period decreased from the initial target concentration of 1,000 mg/l (as nitrogen) to less than 5 mg/l, while sulfate concentrations also demonstrated reductions. The evidence of this process was also confirmed by the elevated anaerobic hydrocarbon degrading bacteria (anhdb) population and the microbial diversity demonstrated through DNA-profiling technology. Compound specific isotope analysis (CSIA) data also provided an additional line of evidence to support the success of the EAB processes with a relative change in the δ 13 C values of the PHC constituents. INTRODUCTION Oxygen is generally depleted in petroleum hydrocarbon (PHC) impacted groundwater; therefore, aerobic degradation processes are limited to the fringes of a PHC plume. Anaerobic processes (degradation that relies upon electron acceptors other than oxygen) will account for most of the biodegradation that occurs within the PHC plume (Bauer et al., 2009). The thermodynamic rates of anaerobic biodegradation follow an order of favorable electron acceptor availability: nitrate (NO 3 - ) > manganese (Mn +4 ) > ferric iron (Fe +3 ) > sulfate (SO 4-2 ) > carbon (Wiedemeier et al., 1999). As a result of macronutrients (nitrogen and phosphorus) uptake by nitrate reducing bacteria, low macronutrient contents limit anaerobic biodegradation processes including most common nitrate and sulfate reductions (Wiedemeier et al., 1999). Our previous remediation practices have indicated that the addition of nitrate and phosphorus to groundwater, where sulfate concentrations were low, resulted in the anaerobic biodegradation of toluene, ethylbenzene, and xylenes; however, benzene appears to demonstrate a lower degradation rate under nitrate-reducing conditions. Some researchers suggest that benzene is biodegradable under sulfate-reducing conditions (Loveley et al., 1995). However, benzene degradation rates appeared to be relatively low at the study site where naturally elevated sulfate was present at an average concentration

2 of 23,000 mg/l sulfate reducing conditions would have been anticipated to be the dominant anaerobic mechanism. A pilot-scale study was completed, following multi-phase vapour extraction (MPVE) source reduction, which consisted of injecting nitrate and phosphorus into the subsurface that contained abundant levels of naturally-occurring sulfate. The addition of nitrate and phosphorus into the sulfate-containing groundwater was expected to induce nitrate reduction and subsequently stimulate sulfate reduction. Combined nitrate and sulfate reducing conditions were postulated to be capable of removing PHC constituents more effectively and possibly increasing the removal efficiency of benzene. SITE DESCRIPTION AND METHODS FIGURE 1. A site plan showing the study site with the remediation system, surrounding land use, and boreholes and groundwater monitoring wells. The study site located in Saskatchewan, Canada, as shown in Figure 1, was impacted by PHCs originating from a retail fueling facility located directly to the northeast and a former bulk fuel facility located to the northwest. The retail fueling facility is composed of two pump islands, one underground storage tank (UST) and one aboveground storage tank (AST). The former bulk fuel facility previously contained six ASTs. Numerous environmental investigations conducted at these two facilities have indicated that PHC migrated towards the off-site area and impacted the study site. A comprehensive environmental investigation was conducted in June The stratigraphy beneath the study site consists of fine grained silt and clay with moderate plasticity. Moist sand seams are interbedded within the silt and clay between approximately 1.2 m below grade level (mbgl) and 5.2 mbgl. Adsorbed-phase PHC constituents were identified at levels exceeding the Saskatchewan vapour inhalation criteria. PHC remediation was required to mitigate potential human health and ecological risks imposed by the elevated PHC concentrations. A MPVE system was connected to each of the recovery wells (RWs) located across the study site. The MPVE system operated for a total of 1,147 hours in September and October As a result of the MPVE system operation, a total of 9,900 L of PHCs was recovered and was comprised of 82% vapour phase and 18% enhanced aerobic biodegradation phase. The EAB program was conducted to facilitate the anaerobic PHC degradation at the study site once the MPVE system was shut down for the winter months.

3 A solution of approximately 420 g/l of potassium nitrate (KNO 3 ) and 75 g/l of triethylphosphate (TEP, (C 2 H 5 ) 3 PO 4 ) was injected under 20 psi to 30 psi pressure into 25 RWs across the study site in October Approximately 5 g/l of sodium carbonate (Na 2 CO 3 ) (ph 10.3) was also added to the solution as a buffering agent. Approximately 2% Ivey-sol 103 surfactant was also injected to desorb PHCs from the soil matrix and transfer the desorbed PHC into groundwater, where it could be more easily remediated by the applied process. Two groundwater monitoring wells, S09-04 (located at the center of the injection area) and S09-10 (located at the edge of the injection area), were selected to be representative of soil and groundwater quality and used for performance monitoring. The monitoring wells were evaluated for water quality parameters (i.e., temperature, electrical conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), and ph) using a QED Environmental Systems Micropurge (MP20) water quality monitoring system with flow cell. The water quality parameters were allowed to stabilize prior to sample collection from the purge stream, except when drawdown was experienced at the 0.1 L/min flow rate. PHC constituents (benzene, toluene, ethylbenzene, xylenes (BTEX) and PHC Fractions F1 (C 6 to C 10 minus BTEX) and F2 (C >10 to C 16 )), general chemical parameters, total and dissolved metals, microbial DNA profiling, anaerobic hydrocarbon degrading bacteria (anhdb), and compound specific isotope analysis (CSIA) of benzene and toluene were analyzed for the collected groundwater samples. In order to assess the soil attenuation, two boreholes S10-01 and S10-02 were drilled approximately 0.5 m north of boreholes S09-04 and S09-10, respectively, using direct push methods to a depth of approximately 6 mbgl. Representative soil samples were collected from the continuous soil cores during the direct push process and analyzed for PHC constituents to compare pre and post-remediation soil conditions. RESULTS AND DISCUSSION Dissolved-Phase Petroleum Hydrocarbon Removal. Changes in the concentrations of dissolved-phase cumulative BTEX and benzene in monitoring wells S09-04 and S09-10 with time are shown in Figure 2 and Table 1. Figure 2 indicates that BTEX has been biodegraded more significantly at S09-04 than at S Benzene concentrations in S09-04 decreased from 15.4 mg/l to 0.68 mg/l; whereas benzene concentration in S09-10 decreased from 16.9 mg/l to 2.81 mg/l (Figure 2 and Table 1). The geochemical environment surrounding groundwater monitoring wells S09-04 and S09-10 are similar, except orthophosphate concentration (6.13 mg/l in S09-04 and 0.15 mg/l in S09-10, respectively) (Table 2). Thus, bioavailable phosphorus (inorganic orthophosphate) appears to play an important role in the rate of the PHC biodegradation in the case electron acceptors (i.e., nitrate and sulfate) being supplied at sufficient concentrations. It is acknowledged that the MPVE system will have played a role in the dissolved phase hydrocarbon reductions; however, the proportion related to mechanical recovery methods is not known. The data does confirm that hydrocarbon reductions continued after the MPVE was shut down, which is demonstrated by the reductions between April 2010 and June 2011 (Figure 2). These reductions are considered to be attributed to the success of the EAB process.

4 FIGURE 2. Changes in benzene and BTEX concentrations in groundwater samples collected from groundwater monitoring wells S09-04 and S TABLE 1. Concentrations of benzene, toluene, ethylbenzene, and xylenes in groundwater samples collected in June 2009, April 2010 and July Sampling Date Benzene (mg/l) Toluene (mg/l) Ethylbenzene (mg/l) Xylenes (mg/l) BTEX (mg/l) S09-04 June 30, April 30, July 30, June 12, S09-10 June 30, April 30, July 30, June 12, Adsorbed-Phase Petroleum Hydrocarbon Removal. Borehole S10-01 was drilled approximately 0.5 m north of borehole S09-04, which was completed to obtain representative post-injection data for the soil conditions in the area of S Only PHC Fraction F2 of the soil sample collected at a depth of 3.86 mbgl within borehole S10-01 was identified at a concentration exceeding the applicable remediation criterion. At the sampling depth of approximately 4 mbgl, removal efficiencies of benzene, toluene and ethylbenzene were 96.7%, 98.7%, and 91.9%, respectively, when the adsorbed-phase PHC concentrations of S10-01 are compared to those of S09-04 (Figure 3a). Removal efficiencies of xylenes and PHC Fraction F1 were 74.3% and 38.2%, respectively; while PHC Fraction F2 concentration is increased from 71 mg/kg to 233 mg/kg (Figure 3a). The increase in PHC F2 concentrations may be attributed to the sampling variability combined with the relatively low concentration. At the sampling depth of approximately 4.5 mbgl, removal efficiencies of BTEX were 93.3%, 99.3%, 97%, 93.9%, respectively; and removal efficiencies of PHC F1 and F2 were 94.2% and 81.3%, respectively (Figure 3b).

5 FIGURE 3 Adsorbed-phase PHC concentrations of soil samples collected at (a) sampling depth of approximately 4 mbgl from boreholes S09-04 and S10-01, and (b) sampling depth of approximately 4.5 mbgl from boreholes S09-04 and S PHC constituents of the soil samples collected from borehole S10-02 were identified at concentrations below the applicable remediation criteria. Removal efficiencies of benzene, ethylbenzene, xylenes, and PHC Fraction F1 were 76.4%, 94.1%, 97.6%, and 97.3%, respectively. Concentrations of toluene and PHC Fraction F2 were identified at levels below the laboratory detection limits following remediation, which represents a decrease from initial concentrations (Figure 3c). FIGURE 3c Adsorbed-phase PHC concentrations of soil samples collected from boreholes S09-10 and S Multiple Lines of Evidence for Enhanced Anaerobic Bioremediation Nitrate Reduction and Sulfate Reduction. Groundwater sampling was not completed immediately following the October 2009 nitrate and TEP injection. However, based on the amount of the injected nitrate, a theoretical nitrate concentration of 1,000 mg/l was expected across the focus area. Nitrate concentration was reduced to the level below detection limit within six month of the nitrate injection, as presented in Table 2. Sulfate concentrations at S09-04 and S09-10 decreased by approximately 4,300 mg/l and 7,600 mg/l within 20 months of the EAB process. Thus, combined nitrate and sulfate reductions are believed to have occurred. Nitrate reduction and sulfate reduction could occur sequentially in the thermodynamic order of nitrate>sulfate. Nitrate and sulfate may have also been consumed simultaneously by the abundance of sulfate reducing bacteria

6 (SRB) in the study site (Westermann and Ahring, 1987; McGuire et al., 2002). A substantial increase in the orthophosphate concentrations was identified in June 2011, after 20 months of injecting the TEP. The substantial orthophosphate increase indicates that more than 12 months are need for TEP breakdown into orthophosphate. TABLE 2. Changes in concentrations of nitrate, orthophosphate and sulfate in S09-04 and S09-10 groundwater samples Sampling Date Nitrate as N (mg/l) Orthophosphate (mg/l) Sulfate (mg/l) S09-04 June 30, April 30, 2010 < July 30, June 12, 2011 < S09-10 June 30, April 30, 2010 < June 12, 2011 < DNA Profiling and anhdb Counting. A groundwater sample collected from a groundwater monitoring well TH was utilized as a control sample for DNA profiling analysis. The monitoring well TH is located approximately 50 m northwest (upgradient) of the study site, considered as one of the source areas (shown in Figure 1) but not affected by the EAB program. Groundwater samples collected from S09-04 and S09-10 show the highest amount of diversity (i.e., the greatest number of individual bands which represents one species) and the highest intensity of a particular band (shown by the blue arrow in Figure 4), which indicates the relative amount of the species within the population. S09-04 S09-10 TH FIGURE 4. DNA profiling of groundwater samples collected from monitoring wells S09-04, S09-10 and TH

7 The microbial diversity of groundwater samples S09-04 and S09-10 is also illustrated by anaerobic hydrocarbon degrading bacteria (anhdb) analysis. Bacteria which carry one of the functional genes for anaerobic hydrocarbon degradation were named as anhdb. The results of anhdb analysis are summarized in Table 2. Population of anhdb in groundwater samples S09-04 and S09-10 are MPN/mL and MPN/mL, respectively; whereas the anhdb population in the control groundwater sample TH is less than 2 MPN/mL. The elevated anhdb population and microbial diversity could be attributed to the native presence of high sulfate concentration and the addition of macronutrient nitrate and phosphorus. TABLE 2. Population of hydrocarbon degrading bacteria in groundwater samples collected from monitoring wells S09-04, S09-10, and TH Sample Anaerobic Hydrocarbon Degrading Bacteria (MPN/mL) S S TH <2 Compound Specific Isotope Analysis. Traditional methods used to confirm bioremediation in the field included monitoring decreases in PHC concentrations and electron acceptors, and increases in microbial community. In recent years, CSIA has gained attentions as a tool for characterizing and assessing in-situ bioremediation of PHC in the contaminated aquifers. The stable isotopic carbon are normally reported in per mil ( ) units from a reference standard using conventional δ notation (Eq. 1) where R = 13 C/ 12 C (Hunkeler et al., 2008). Organisms preferentially use the lighter isotope species due to the lower energy cost, resulting in fractionations between the heavier isotope and the lighter isotope. The positive shifts in the ratio of 13 C to 12 C (δ 13 C values) provides direct evidence to substantiate a claim of in-situ biodegradation, instead of physical attenuation (i.e., dispersion and adsorption) (Song et al., 2002). A positive shift in δ 13 C benzene and δ 13 C toluene in S09-10 (from to and from to -28.9, respectively) was reported and is presented in Table 3. Similarly, an enrichment of δ 13 C toluene (from to ) occurred in S However, a negative shift in δ 13 C benzene (from to ) was reported in S09-04 for some unknown reasons. Additional CSIA data is needed in the future to further evaluate the relationship between the CSIA data and the PHC biodegradation. TABLE 3. δ 13 C ( Vienna Peedee Belemnite (VPDB)) analysis of dissolved benzene and toluene in groundwater samples S09-04 and S09-10 collected in April and October, Sample δ 13 C benzene ( VPDB) δ 13 C toluene ( VPDB) April 30, 2010 S S September 22, 2010 S S

8 CONCLUSIONS The treatment approach applied to the study site through the application of mechanical source removal utilizing MPVE followed by an EAB process has provided successful reduction of PHC concentrations in soil and groundwater. The in-situ EAB process is postulated to combine nitrate reduction with sulfate reduction, which is induced by nitrate reduction, naturally elevated sulfate and residual phosphorus. In addition to the decrease in concentrations of PHC constituents and electron acceptors (i.e., nitrate and sulfate), microbial technology (i.e., DNA profiling and anhdb counting) and CSIA of benzene and toluene provided further supporting evidence for the EAB process. The EAB process, integrating nitrate reduction with induced sulfate reduction, will continue to be studied and will possibly provide a novel and cost effective approach to remediate PHC-impacted sites at many sites with abundant levels of naturally occurring sulfate. REFERENCES Bauer, R.D., Rolle, M., Bauer, S., Eberhardt, C., Grathwohl, P., Kolditz, O., Meckenstock, R.U., Griebler, C Enhanced Biodegradation by Hydraulic Heterogeneities in Petroleum Hydrocarbon Plumes. Journal of Contaminant Hydrology 105: Hunkeler, D., Meckenstock, R.U., Lollar, B.S., Schmidt, T.C., Wilson, J.T A guide for assessing biodegradation and source identification of organic ground water contaminants using compound specific isotope analysis (CSIA). EPA 600/R-08/148. U.S. Environmental Protection Agency. Lovley, D.R., Coates, J.D., Woodward, J.C., Phillips, E.J.P Benzene oxidation coupled to sulfate reduction. Applied and Environmental Microbiology 61: McGuire, J.T., Long, D.T., Klug, M.J., Haack, S.K., Hyndman, D.W Evaluating behavior of oxygen, nitrate, and sulfate during recharge and quantifying reduced rates in a contaminated aquifer. Environmental Science & Technology 36: Song, D.L., Conrad, M.E., Sorenson, K.S., Alvarez-Cohen, L Stable carbon isotope fractionation during enhanced in situ bioremediation of trichloroethene. Environmental Science & Technology 36: Westermann, P. and Ahring, B.K Dynamics of methane production, sulfate reduction, and denitrification in a permanently waterlogged Alder Swamp. Applied and Environmental Microbiology 53: Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E Technical protocol for implementing intrinsic remediation with long-term monitoring for natural attenuation of fuel contamination dissolved in groundwater Volume I, Air Force Center for Environmental Excellence, San Antonio, Texas.